Typical Chain Values. Sprockets

Three essential factors characterize a chain’s suitability for use as a timing chain:
- Breaking strength
- Endurance, Fig. 7-231
- Wear resistance

One cause that might be responsible for failure is exceeding the static or dynamic breaking load.

Fig. 7-231. Fatigue strengths for roller and bushed roller chains

Particularly in timing drives, one does not encounter uniform loading. Pulsating loading on the chain results from fluctuating torques at the camshaft and the injection pump (in diesel engines, for example), nonuniform camshaft rotation, and pulsating longitudinal chain forces caused by the polygonal effect. Here the chain’s fatigue strength must never be exceeded since the number of load alterations during an engine’s service life is in all cases greater than 10⁸.

In today’s engines with their precise timing, minimal stretching due to wear, at from 0.2% to 0.5% of chain length at up to 250000 km in service, can be attained.

A chain timing system with its mass, stiffness, and damping represents a system capable of oscillation, having several degrees of freedom (Fig. 7-232). In response to excitation by the camshaft, crankshaft, injection pump, etc., this can cause resonance effects that result in extreme loading of the timing drive system.

Fig. 7-232. Typical chain values: Stiffness and mass

Engineering measures make it possible to increase stiffness in the chain while retaining its specific mass. This shifts resonance points toward the higher frequencies.

Sprockets. The shapes of the teeth in sprockets intended for use with roller chains, bushed roller chains, and silent chains are standardized (DIN 8196). Proper tooth profile is just as important to reliable operation of the timing system as, for example, the chains’ wear resistance.

Usually sprockets with the widest tooth gap are used. This makes possible, because of the short teeth and the wider gap between teeth, uninterrupted engagement and disengagement of the chain even at higher chain speeds.

Fig. 7-233. Sprockets

Depending on the amount of space available and the particulars of the application, pulleys or sprockets with one or two rows of teeth may be used (Fig. 7-233). The selection of the materials depends on the timing drive system parameters, the operating conditions, and the amount of power to be transferred.

Carbon steel, alloyed steels, and sintered materials are used for the sprockets.

The materials used for precision punched sprockets include C 10 or 16MnCr5 for sprockets made with cutting processes, and D 11 for sintering processes, together with the heat treatment suitable for each particular material.